Obesity, and more specifically insulin resistance is characterized by impaired energy expenditure associated adaptive thermogenesis. Adaptive thermogenesis in animals involves activation of brown adipose tissue (BAT), a tissue previously thought to completely regress in adult humans. Biopsy-confirmed BAT in adult humans has revived interest in discovering whether dysfunction in regulation and/or activity of BAT contributes to the pathogenesis of obesity. New methodologies are needed to increase our knowledge of BAT in humans. The purpose of this application is to test a new, mechanistically driven approach using positron emission tomography (PET) imaging of BAT in humans. BAT is strongly innervated and regulated by the sympathetic nervous system. We propose to use a structural feature of the sympathetic nervous system, and image the norepinephrine recycling component, designated the norepinephrine transporter (NET). For this purpose, we will utilize a recently developed, highly selective NET ligand for PET imaging, ((S, S)-[11C]O-methylreboxetine) ([11C]MRB). PET imaging NET has the advantage over [18F]-2-fluoro-deoxy-D-glucose ([18F]FDG) currently in use, because unlike [18F]FDG, it is more specific, and should allow detection of non-stimulated BAT. Our preliminary ex vivo and in vivo imaging studies in rodents and a human subject have shown that [11C]MRB can efficiently label BAT at both room temperature and mild cold conditions;in contrast, [18F]FDG labeling of BAT occurs only under mild cold conditions. The results from our preliminary blocking study with unlabeled MRB also demonstrated the specificity and saturability of [11C]MRB for imaging BAT. These data support our general hypothesis that the [11C]MRB PET imaging approach will provide a non-stimulated target for BAT that is proportional to BAT mass. Data also suggest NET up-regulation with activation of adaptive thermogenesis due to environmental cold exposure. Our aims are divided into both animal, and human experiments. The animal work will directly compare the [11C]MRB and [18F]FDG tracers in awake rats under room temperature and mild cold (40C) conditions for their relationship to interscapular BAT dimensions, mass, and intensity of label. Moreover, we will examine interscapular BAT during in vivo PET imaging after both environmental conditions for [11C]MRB labeling kinetics. Human work will both validate the [11C]MRB PET strategy against the current gold standard of [18F]FDG labeling, using both room temperature and mild cold exposure, as well as provide a preliminary gender comparison of BAT, also using the [11C]MRB PET strategy with room temperature and mild cold exposures. By targeting a primary regulatory system component of BAT, NET, we are establishing a basis for future mechanistic studies of BAT function/dysfunction in obesity and diabetes, as well as for therapeutic approaches for these disorders.